Biomolecules bound to catalytic inorganic particles, immunoassays using the same

ABSTRACT

Catalytic colloidal metal particles bound to a biomolecule such as an antibody, avidin, or streptavidin are useful for detecting the presence of the biomolecule in an assay such as an immunoassay.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to biomolecules which are bound tocatalytic inorganic particles, immunoassays which utilize suchbiomolecules, and kits for carrying out such immunoassays.

2. Discussion of the Background

The detection of trace amounts of biologically significant compounds,such as steroids or drugs of abuse, is often accomplished quickly andinexpensively by the employment of an immunoassay. Such an assay relieson an immunogenic recognition of the substance in question followed bythe amplification of that recognition. Enzymes are widely used inimmunoassays as the amplifier of the antibody-antigen recognition event.One of the most common types of immunoassays is the Enzyme-LinkedImmunosorbant Assay (ELISA).

ELISA may be preformed in a number of different ways. The two mostcommon are the competitive mode and the sandwich assay. In a competitivemode ELISA, a surface, usually either a polystyrene plate or anitrocellulose membrane, is coated with a capture antigen. Thesesurfaces are normally chosen because they bind protein non-specifically.Therefore, if the antigen is not a protein, it may be covalently linkedto a carrier protein and bound to the surface without further chemistry.After the antigen is bound, the remaining binding sites on the surfaceare blocked with another protein. Then the test fluid and enzyme-labeledantibody are added. If no antigen is in the test fluid, all the labeledantibody will bind to the antigen adsorbed on the surface. Conversely,if antigen is present in the test fluid, the antigen will block thebinding sites on the enzyme-labeled antibody and prevent it from bindingto the antigen adsorbed on the surface. The surface is washed to removeunbound materials, and a substrate is added for the enzyme. The enzymecatalyzes a reaction in which the substrate reacts to form a coloredmaterial which can be quantitatively observed with a spectrophotometer.The intensity of the color produced is proportional to the enzymeactivity and the amount of antibody bound, which is inverselyproportional to the amount of antigen in the test fluid.

In a sandwich assay ELISA, an antibody that recognizes part of theantigen is bound to a surface. Since antibodies are proteins, this isreadily accomplished by allowing the surface to contact a solution ofthe antibody. As in the competitive ELISA, the remaining sites on thesurface are blocked with another protein. The test fluid is then added.If an antigen is present in the test fluid, the antibody on the surfacewill capture the antigen. Then a second, enzyme-labeled antibody, whichrecognizes a different part of the antigen than the first antibody, isadded. The second antibody will then bind to the antigen which iscaptured on the surface. After washing the surface to remove any unboundmaterials, a substrate for the enzyme is added and the color produced isobserved spectrophotometrically. In this form of an ELISA, the signal isdirectly proportional to the concentration of the antigen in a testsample. Such a sandwich assay is widely used in the commercial arena forhome pregnancy tests.

In either type of ELISA, the enzyme acts as the amplifier of theantigen-antibody reaction. That is, a color or other signal, such aslight from some chemiluminescent reaction, is produced that can beobserved macroscopically. Without this amplification step, thesensitivity of an immunoassay would be orders of magnitude less.

Several problems occur in the use of enzymes as amplifiers inimmunoassays. They are:

1. Any change in enzyme activity will affect the precision of the assay.For example, loss of half of the activity of the enzyme in a competitiveELISA may produce a false positive since less signal indicates thepresence of the test substance. Since enzyme activity is sensitive tostorage conditions, enzymes must be kept either refrigerated, freezedried or both. Also, controls must be performed to constantly test theactivity of the enzyme. Inevitably, the shelf-life is limited by thestability of the enzyme.

2. Enzymes are expensive. Being derived from living sources, theyrequire substantial processing costs. The least expensive enzyme, on anactivity basis, is Horseradish Peroxidase which is not surprisingly themost common enzyme used in ELISAs. However, even Horseradish Peroxidasecosts about $5/mg or $5000/g, a cost which is about 450 times the costof gold. Fortunately, very little enzyme is necessary for each assay.

3. The labeling of antibodies with enzymes is often a quite laboriousprocedure as one must ensure that little unbound enzyme is present. Ifsignificant amounts of unbound enzyme are present or significant amountsof unlabeled antibody are present, the sensitivity of the ELISA isreduced.

4. Enzymes are often heterogeneous materials due to their isolation fromnatural sources. Therefore, characterization of enzyme-antibodyconjugates can be difficult.

Enzymes are also used for detection of the hybridization of DNA or RNAto its complimentary strand, often in conjunction with amplification ofthe DNA or RNA target by the polymerase chain reaction (PCR). Thesereactions are widely employed for DNA fingerprinting, and the detectionof genetic defects, viruses and bacteria. Because the PCR reactionrequires heating and cooling of the reaction mixture to causedenaturation of the DNA, the common enzymes such as peroxidase oralkaline phosphatase cannot be added to the reaction mixture until afterthe amplification reactions occur. This limits some of the proceduresthat can be preformed. Catalytic particles do not have this limitationand therefore give more flexibility to the detection of nucleic acids.

Colloidal metals have been employed in immunoassays previously. Mostly,they consisted of either colloidal iron or gold (M. Horisberger,"Colloidal Gold: A Cytochemical Marker for Light and FluorescentMicroscopy and for Transmission and Scanning Electron Microscopy",Scanning Electron Microscopy, pp. 19-40 (1981); and Martin et al,"Characterization of Antibody Labelled Colloidal Gold Particles andTheir Applicability in a sol Particle Immunoassay, SPIA", J.Immunoassay, vol. 11, pp. 31-48 (1990)). However, in either case, themetals were only chosen for their color, i.e., their presence isdetermined only by their color or electron density under an electronmicroscope. Both the color and electron density are directlyproportional to the mass of the metal colloid, not their catalyticactivity. Thus a relatively large amount of material is necessary to beobserved and they can only compete in sensitivity to enzyme-typeamplifiers of the antibody-antigen reaction when the signal is furtheramplified by an instrument such as an electron microscope.

Similarly, the use of colloidal gold and colloidal silver as markers inhistochemistry has also been reported. (Lucocq and Roth, "Colloidal Goldand Colloidal Silver-Metallic Markers for Light MicroscopicHistochemistry", Techniques in Immunochemistry, vol. 3, pp. 203-236(1985)). Again, the colloidal particles were not detected on the basisof any catalytic activity. More recently amplification of gold colloidshas occurred via a process very similar to photography. The goldcolloids act as nucleation sites for the precipitation of silver, whichis the colorimetric material (see p. 273 of the 1993 BioRad LifeSciences Research Product Catalog, Hercules, Calif.).

Stable colloidal rhodium (0) suspensions have been reported to catalyzethe hydrogenation of liquid alkenes in biphasic systems under mildconditions (Larpent et al, "New Highly Water-Soluble SurfactantsStabilize Colloidal Rhodium (0) suspensions Useful in BiphasicCatalysts", J. Molecular Catalysis, vol. 65, pp. L35-L40 (1991)).However, there is no report of such colloidal rhodium particles beingbound to a biomolecule, such as an antigen.

The oxidation of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) byhydrogen peroxide is a chemiluminescent reaction known to be catalyzedby colloidal platinum (Albrecht, Z. Phys. Chem., vol. 136, p. 321(1928)). However, there is no report of such colloidal platinum beingbound to a biomolecule.

Tris(2,2'-bipyridine)ruthenium II has been used as a peroxide-producingreplacement for an enzyme label (Ismail and Weber,"Tris-2,2'-Bipyridineruthenium-II as a Peroxide-Producing Replacementfor Enzymes as Chemical Labels", Biosens. Bioelectronics, vol. 6, pp.698-705 (1991). However, the hydrogen peroxide is produced by photolysiswith such compounds, and accordingly, the use of such labels in an assayrequires the use of photolysis equipment.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide novelcompounds which can be used for the immunoassay of an analyte which donot suffer from the above-described drawbacks.

It is another object of the present invention to provide compounds inwhich a biomolecule is bound to a catalytically active moiety which isstable on long-term storage.

It is another object of the present invention to provide compounds inwhich a biomolecule is bound to a catalytically active moiety which isstable at elevated temperatures.

It is another object of the present invention to provide compounds whichare catalytically active at conditions at which biomolecules are stableand which are easily accessible.

It is another object of the present invention to provide compounds whichare catalytically active at room temperature, near neutral pH, and inaqueous media.

It is another object of the present invention to provide a novelimmunoassay utilizing a compound in which a biomolecule is bound to acatalytically active moiety which is stable on long-term storage.

It is another object of the present invention to provide a novelimmunoassay utilizing a compound in which a molecule is bound to acatalytically active moiety which is stable at elevated temperatures.

It is another object of the present invention to provide novel kits forcarrying out such immunoassays.

These and other objects, which will become apparent during the followingdetailed description, have been achieved by the inventors' discoverythat a catalytic particle based on a metal colloid possesses catalyticactivity equivalent to that of an enzyme. Unlike enzymes, metals do notlose activity over time and no special handling is necessary. Sincemetals are readily obtained from their ores, even precious metals, suchas platinum, cost about $12/g or 400 times less than the typically usedenzymes. Some catalytic particles will adsorb antibodies and otherproteins non-specifically which makes attachment easy. In somepreparations, the catalytic particles are significantly larger than theantibodies or proteins or more dense. Therefore, the particles may beseparated from unbound antibody by any number of physical techniquessuch as size exclusion chromatography or centrifugation.

This disclosure describes the preparation of colloidal metal catalyticparticles for the replacement of enzyme amplification of molecularrecognition, their labeling with proteins, their purification, their usein immunoassays, and kits for carrying out such immunoassays.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 schematically illustrates a competitive mode assay according tothe present invention;

FIG. 2, schematically illustrates a sandwich mode assay according to thepresent invention; and

FIG. 3 schematically illustrates a DNA assay according to the presentinvention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Thus, in a first embodiment, the present invention relates to novelcompounds in which a biomolecule is bound to a catalytically activecolloidal metal particle. The criteria for selection of the colloidalmetal is that a colloid of the metal should be easily prepared, thecolloid should be capable of being labeled with a protein, the colloidshould not coagulate while in solution, and the colloid should becapable of catalyzing a colorimetric or chemiluminescent reaction.Accordingly, the colloidal metal particle may suitably comprise anymetal which is catalytically active as a colloidal particle and meetsthose criteria. Preferably, the metal is Ag, Pt, or Pd. Most preferably,the metal is Pt.

It should be understood that the colloidal metal particle may comprisemore than one metal. Thus, the colloidal metal particle may comprise twoor more metals which are catalytically active as a colloidal particle,e.g., a colloidal particle of Pt and Pd. Alternatively, the colloidalmetal particle may comprise an inactive or inert metal, such as gold, inaddition to the catalytically active metal.

In the context of the present invention, the term "catalytically active"means the ability to catalyze any reaction which may conveniently usedas the signal amplification in an immunoassay. Examples of suchreactions include:

1. The transfer of hydrogen from hydrogen donors (HD) to H₂ O₂ :

    HOOH+2DH→2H.sub.2 O+2D

D=hydrazine, 1,2-dihydroxybenzene, diakyl-N,N'phenylenediamine,1,3-dihydroxybenzene, 1,4-dihydroxybenzene, 1,2,3-trihydroxybenzene,ο-phenylenediamine, ο-methoxyphenol, coniferol, leucomalachite green,benzidine, ascorbic acid, guaiacol, diaminobenzidine, 5-aminosalicylicacid, ο-dianisidine, the diammonium salt of2,2'-azino-di-(3-ethyl-benzthiazoline sulfonate-6),3-methyl-2-benzothiazolinone hydrazone, 3-(dimethylamino)benzoic acid,and ο-tolidine.

Platinum and palladium are widely used as catalysts in organicchemistry. Since these particles stay in aqueous solution, there is theopportunity to use these materials in that solvent. Aqueous solutionsoffer advantages in cost of the solvent and disposal of the waste.Several systems have been studied in the past for the use of colloidalplatinum in organic reactions (c.f. Gemelin Handbuch fur AnorganischenChemie, Platin A3, pp. 429-430). However, with the discovery of Adam'scatalysis, experimentation with colloids appears to have beendiscontinued.

Suitably, the colloidal metal particle is small enough thatagglutination will not occur and the colloidal particles will remain inthe aqueous phase for times as long as weeks or months. Although theupper size limit will depend, in part, on the storage conditions andidentity of the metal, it has been found for Pt that good results areachieved if the colloidal particles are smaller than 50 nm in theirgreatest dimension, preferably less than 10 nm in their greatestdimension. Suitably, the colloidal particles have a smallest dimensionof at least about 1 to 5 nm.

The colloid metal particles may be prepared by any conventional method.A number of methods for producing colloidal particles which are suitablefor the present invention have already been reported, and these aredescribed below.

A number of reducing agents for gold are known and include citrate (G.Frens, "Controlled Nucleation for the Regulation of the Particle Size inMonodisperse Gold Suspensions", Nature Physical Science, vol. 241, pp20-22 (1973)), formate, ascorbate, formaldehyde, phosphorus, ethanol,and tannic acid (M. Horisberger, "Colloidal Gold: A Cytochemical Markerfor Light and Fluorescent Microscpy and for Transmission and ScanningElectron Microscopy", Scanning Electron Microscopy, pp. 19-40 (1981)).Less work has been done on platinum. Gutbier (A. Gutbier, "InorganicColloids", Z. Chem. Ind. Kolloide, vol. 5, pp. 46-52 and 105-109)prepared a colloidal platinum and palladium solution with hydrazine, butno catalytic reactions were reported.

Brintzinger reduced a number of metals with ascorbic acid and relatedmaterials (H. Brintzinger, "Ascorbic and Isoascorbic Acids as ReducingMedia in the Formation of Colloid Disperse Solutions of Gold, Palladium,Platinum, Silver, Selenium, Tellurium, Molybdenum Blue and TungstenBlue", Kolloid-Z., vol. 78, pp. 22-23 (1937); c.f. Chemical Abstracts,vol. 31 (1937)). Fillippov and Gushchina have employed formicacid/sodium formate (M. P. Filippov and L. F. Guschcina, "Determinationof Colloidal Platinum by a Kinetic Method", Journal of AnalyticalChemistry USSR, vol. 19, pp. 441-443 (1964)) to form colloidal platinumand determined the platinum concentration by its catalytic effect on thereduction of phosphomolybdenum blue at pH 2.7.

Gillet also reduced platinum with formate and determined its presence bythe catalytic reduction of methylene blue in acid media (C. Gillet, Jr.,"Separation, par Amalgamation en Mileiu Homogene, et DosageCatalymetrique du Platine avec le Systeme or-Formiate-Bleu deMethylene", Mikrochimica Acta [Wien], II, pp. 467-477 (1977)). Pilipenkoand Terletskaya prepared colloidal platinum and palladium and determinedthat they produced chemiluminescence with lucigenin (A. T. Pilipenko andA. V. Terletskaya, "Catalytic Effect of the Platinum Metals in TheirColloidal State on the Reaction Between Lucigenin, Hydrazine, andOxygen", Journal of Analytical Chemistry USSR, vol. 28, pp. 1004-1008(1971)). In no case were these particles attached to biomolecules oremployed in immunoassays, and in most cases the reaction conditionsemployed are quite severe. All of the references cited in this and thepreceding paragraphs describing the production of colloidal metalparticles are incorporated herein by reference.

Some of the classical reducing agents produce colloidal particles thatare too large for immunoassays or coagulate during formation. Thus, alarge series of buffers and reducing conditions were tested as inExample 1. Some of the conditions and results are shown in Table 1.

The colloidal metal particle is most conveniently bound to thebiomolecule by physical adsorption. Typically, the colloidal particle ina medium comprising phosphate buffer at a pH near the isoelectric pointof the biomolecule is incubated with the biomolecule. The incubation istypically carried out at room temperature for a time of 5 minutes toseveral days, preferably about 1 hour. The colloidal metal tobiomolecule ratio is adjusted by a number of factors. The products arerun by electrophoresis on an agarose gel, and the shift in mobilitynoted when the colloidal metal is bound to the biomolecule.Alternatively, immunoassays are run with the product and theconcentration of the biomolecule is chosen to produce maximumsensitivity. The metal particles will aggregate if the biomolecule isnot coated sufficiently on the surface. This aggregation takes severalhours to days at room temperature. However, it may be accelerated bycentrifugation of the colloids at 50,000 Gs for 1 hour. The pellet, thatresults will easily resuspend if the colloidal particles aresufficiently coated with the biomolecule. This may serve as a thirdmethod to determine the optimum ratio of biomolecule/colloid.

For example, good results have been achieved by the following process. Apreparation of colloidal Pt particles is prepared by adding 50 μl of an8 wt. % solution of platinum chloride in water (prepared from H₂ PtCl₆,Pt content˜4 wt. %) to 10 ml of H₂ O in which has been dissolved 300 mgof ascorbic acid and 300 mg of NaHCO₃. The resulting mixture is heatedto near boiling until it turns brown (about 30 min) and then cooled toroom temperature. 100 μl of the resulting colloidal suspension is addedto about 100 μl of buffer and then 1 to 10 μg of protein is added toachieve a final protein concentration of 5 μg/ml to 50 μg/ml, and themixture is incubated for about 1 hour.

Within the context of the present invention, the term biomolecule refersto any molecule which can be used in an immunological assay.Specifically, biomolecules which may be used in the present inventionincluded antibodies (monoclonal and polyclonal), avidin, andstreptavidin, proteins, proteins with a hapten attached, or antigens.

It should also be understood that the colloidal metal particle may bebound to a biomolecule which is not a polypeptide via a molecule whichis a polypeptide. Thus, a colloidal metal particle may be adsorbed on,e.g., avidin which can in turn be bound to biotin, which in turn may becovalently bound to, e.g., a polynucleic acid or a polyribonucleic acid.

The present biomolecules bonded to a catalytically active colloidalmetal particle may be used directly after preparation and while in anaqueous suspension. Alternatively, the biomolecule bonded to thecolloidal particle may be stored in the form of an aqueous solution forvarying periods of time before being used. In another embodiment, thebiomolecule bonded to the colloidal particle is stored as a dry powder,which may be obtained, e.g., by lyophilization.

The present biomolecules which are bound to a colloidal metal particlemay be used in a number of different types of immunoassays. The presentbiomolecules are particularly useful as replacements for theenzyme-linked biomolecules currently used in ELISAs.

ELISAs are discussed in detail in Tijssen, Practice and Theory of EnzymeImmunoassays, Elsevier, N.Y., (1985), which is incorporated herein byreference. ELISAs are also discussed in Engvall et al, Immunochemistry,vol. 8, p.871 (1971); Engvall et al, Methods in Enzymology, vol. 70(1980); U.S. Pat. Nos. 4,558,012; 5,176,999; 5,173,404; Re 31,006; andRe 32,696; all of which are incorporated by reference. Enzymes which areused in activity amplification assays include peroxidase,β-galactosidase, alkaline phosphatase, urease, glucose oxidase,glucoamylase, carbonic anhydrase, and acetylcholinesterase. As notedabove, Horseradish peroxidase is the most widely used enzyme in ELISAs.

FIG. 1 schematically illustrates a competitive mode ELISA utilizing anantibody labelled with a colloidal metal particle. A test fluid, whichmay contain the target antigen (1), is added to a container whichcontains immobilized target antigen (2). Then antibody (3) which bindsto the target antigen and which is labelled with a colloidal metalparticle is added. After incubation and washing, the amount of labelremaining in the container is measured. A higher amount of antigen inthe test sample will result in a lower amount of label detected.

FIG. 2 schematically illustrates a sandwich mode ELISA, according to thepresent invention. A test sample which may contain the target antigen(1) is added to a container which contains immobilized antibody (4)specific for a first epitope on the target antigen. Then antibody (5)which is labelled with a colloidal metal particle and recognizes asecond epitope of the target antigen is added. After incubation andwashing, the amount of label remaining in the container is measured. Ahigher amount of antigen in the test sample will result in a higheramount of label detected.

It will be readily recognized by those of skill in the art that thesemethods are also useful for the detection of antibodies in a test sampleas well as antigens in test sample.

Although the above-described ELISAs are many times useful for obtainingquantitative results, it is often sufficient that the test provide onlyqualitative results. In particular, qualitative results are sufficientfor a home or early pregnancy test.

In a preferred embodiment, the biomolecule will be streptavidin.Typically, the colloidal particle bound to streptavidin will be used inconjunction with an antibody or antigen which is covalently linked tobiotin. The labelling of antibodies and antigens with biotin is wellwithin the abilities of the skilled artisan. Preferably, the colloidalparticle bound to streptavidin is used in conjunction with an antibodybound to biotin. In this way, it is possible to tailor the present assayfor the detection of any desired antigen by judicious choice of theantibody bound to biotin.

The present biomolecules bound to a colloidal metal particle are alsouseful for determining the presence or absence of a specific sequence ofRNA or DNA. The present molecules are particularly useful for detectingthe presence of a target DNA in conjunction with the polymerase chainreaction.

In a first type of DNA or RNA assay, a sequence of DNA or RNA which iscomplementary to the target DNA or RNA is immobilized on a support, andthe immobilized DNA or RNA is treated first with the test sample whichmay contain the target DNA or RNA and then with a sequence of DNA or RNAwhich is labelled with a colloidal metal particle and which will alsohybridize with the initially immobilized DNA or RNA. After incubationand washing, the amount of label is measured. This embodiment isessentially the same as the competitive ELISA described above.

The present biomolecules bound to a colloidal metal particle areparticularly useful for detecting the presence or absence of a targetDNA in conjunction with PCR amplification. This type of assay isschematically illustrated in FIG. 3. A sample which may contain thetarget DNA (6) is incubated in steps (a) and (b) with: DNA polymerase, aDNA probe (7) which is identical to a portion of the target DNA (6) andwhich will be extended by the DNA polymerase and which is labelled with,e.g., biotin (8); and a DNA probe (9) which will hybridize with thetarget DNA (6) but not probe (7) and is labelled with a colloidal metalparticle (10). After incubation and denaturation of the DNA, the singlestranded DNA which contains probe (9) will then hybridize with probe (7)and incubation with DNA polymerase in step (e) will yield a doublestranded DNA which contains both probe (7) and probe (9). Theamplification may be extended by heating the double stranded DNAafforded by step (e) and again incubating in the presence of DNApolymerase. By maintaining an excess of probes (7) and (9) in themixture, it is ensured that the double stranded DNA obtained will belabelled with both biotin (8) and the colloidal metal particle (10).

After the amplification is complete, the reaction mixture is contactedwith immobilized streptavidin (11). Only if the target DNA (6) waspresent in the test sample will the reaction mixture contain doublestranded DNA which contains both probe (7) and (9) and thus is labelledwith both biotin (8) and colloidal metal particle (10). Thus, only ifthe target DNA (6) was present in the test sample will the colloidalmetal particle (10) be bound to the immobilized streptavidin (11) and,thus, detected after washing. It is noteworthy that enzymes are notsuitable as labels for such an assay because they will be denatured bythe heating step required by PCR.

Of course, one of skill in the art will recognize that theabove-described method may be carried out by using complementary bindingsystems other than the biotin-streptavidin system.

In the assays utilizing DNA or RNA labelled with a colloidal metalparticle, the colloidal metal particle will typically be linked to theDNA or RNA by adsorbing avidin or streptavidin on the metal colloidalparticle and allowing the avidin or streptavidin to bind to biotin whichis covalently bonded to the DNA or RNA. The covalent binding of biotinto oligonucleotides is well known in the art and is described in:Chollet et al, Nucleic Acids Res., vol. 13, p. 1529 (185); Wachter etal, Nucleic Acids Res., vol. 14, p. 7985 (1986); Agarawal et al, NucleicAcids Res., vol. 14, p. 6227 (1986); Urdea et al, Nucleic Acids Res.,vol. 16, p. 4937 (1988); Cook et al, Nucleic Acids Res., vol. 16, p.4077 (1988); Landergreu et al, Science, vol. 241, p. 1077 (1988);Mitchell et al, Anal. Biochem., vol. 178, p. 239 (1989); Richterrich,Nucleic Acids Res., vol. 17, p. 2181 1989); Cocuzza, TetrahedronLetters, vol. 30, pp. 2687-6290 (1989); U.S. Pat. No. 4,908,453; Alveset al, Tetrahedron Letters, vol. 30, pp. 3089-3092 (1989); U.S. Pat. No.4,605,735; European Patent Application 202,758; Kempe et al, NucleicAcids Res., vol. 13, p. 45 (1985); U.S. Pat. No. 4,751,313; U.S. Pat.No. 4,711,955; and U.S. Pat. No. 5,128,476; all of which areincorporated herein by reference.

The presence of the biomolecule bound to the colloidal metal particle isdetected by relying on the catalytic activity of the colloidal metalparticle. The catalytic activity of the colloidal particle may bevisualized by means of either a colorimetric (any change in absorbanceof light) or chemiluminescent reaction. For example, the colloidal metalparticle may be contacted with a mixture comprising H₂ O₂ and a suitablehydrogen donor (HD) and the activity and, hence, amount of colloidalmetal particle can be determined by measuring the amount of the reactionproduct, D, colorimetrically. Alternatively, the colloidal metalparticle may be contacted with a mixture comprising a molecule whichwill react with a product catalytically produced by the colloidalparticle to produce light. Examples of such chemiluminescent systemsinclude the luminol system and the lucigenin system, which are explainedin detail in the Examples below. Another suitable system is the oxalateester system described in Beck et al, Anal. Chem., vol. 62, pp.2258-2270 (1990), which is incorporated herein by reference.

In another embodiment, the present invention relates to kits forcarrying out an assay utilizing a biomolecule bonded to a catalyticallyactive colloidal metal according to the present invention. Such kitswill typically contain a first container containing a premeasured amountof a biomolecule bound to a colloidal metal. The kit may also contain asecond container containing a premeasured and known amount of the targetanalyte to serve as a standard for the assay. The kit may furthercontain written instructions for carrying out the assay. The biomoleculebound to the colloidal metal may be contained in the kit in the form ofa dry powder or a liquid suspension of known activity. If thebiomolecule is contained in the kit in the form of a dry powder, thenthe kit may also contain measuring means for preparing a liquidsuspension of known activity, such as a volumetric container.

Other features of the invention will become apparent in the course ofthe following descriptions of exemplary embodiments which are given forillustration of the invention and are not intended to be limitingthereof.

EXAMPLES

I. Preparation of Colloids

Example 1

To a 7 mL vial was added 2 mL of buffer, 5 μl of 8 wt. % platinumchloride in water solution (prepared from H₂ PtCl₆, Pt content˜4 wt. %),100 μl of reducing agent (37 wt. % formaldehyde, 25 wt. % glutaricdialdehyde or 10 mM ascorbic acid) and 10 μl of surfactant (1 mM). Thevial was sealed and heated at approximately 75°-85° C. for 90 minutes inan oven. The vials were removed occasionally and color developmentnoted. However, the reaction was not stopped when the first appearanceof colloid was noted so that the particles could grow larger if theconditions were more optimal. The characterization of the particles isonly qualitative. The size was judged by their tendency to settle. Ifcomplete settling had occurred, the particles were deemed to be large.If only limited settling had occurred after 24 hours, the particles weredeemed to be small with precipitate. If no settling had occurred after24 hours, the particles were deemed to be colloidal. From Table 1, it isevident that a large number of conditions generate colloidal particles.

                                      TABLE 1                                     __________________________________________________________________________    Reduction of Platinum Chloride Under Various Conditions and pHs                                BUFFER                                                                        Phosphate                                                                             Borate                                                                              Tris  Carbonate                                Reducing Agent                                                                          Surfactant                                                                           7 8 9 10.sup.a                                                                        8.6                                                                             9 10.sup.a                                                                        7.2                                                                             8 9.sup.a                                                                         8.3                                                                             9 10                                                                              11.sup.a                           __________________________________________________________________________    Ascorbate None   O S S O B S S C C C O O N N                                            Cationic                                                                             L S S O B S S C C C O O N N                                            SDS    L S O O B S S C C C O O N N                                            Triton X100                                                                          L L L N B S S C C C O O N N                                  Formaldehyde                                                                            None   L S L L L L L C C C L L L L                                            Cationic                                                                             L L L L L L L C C C L L L L                                            SDS    S S L S L L L C C C L L L L                                            Triton X100                                                                          S L L L L L L C C C L L L L                                  Glutaric Dialdehyde                                                                     None   O O O O B B B C C C B O O O                                            Cationic                                                                             O O O O O O O C C C O O O O                                            SDS    O O O O O O O C C C O O O O                                            Triton X100                                                                          O O O O O O O C C C O O O O                                  __________________________________________________________________________     .sup.a The numbers under the Buffer is the pH.                                Key:                                                                          Surfactants:                                                                  Cationic = Hexadecyltrimethylammonium bromide                                 SDS = Sodium dodecylsulfate                                                   Results:                                                                      O = Colloid (Brown solution)                                                  B = Colloid (Black solution)                                                  S = Small Precipitates                                                        L = Large Precipitates                                                        N = No reaction and no color                                                  C = Color but no reaction                                                     H = Hazy solution                                                        

Example 2

300 mg of ascorbic acid and 300 mg of sodium bicarbonate were dissolvedin 10 mL of distilled water. The solution was heated to approximately90° C., and 100 μl of 8 wt. % platinum chloride solution in water(prepared from H₂ PtCl₆, content˜4 wt. %) was added. The heating wascontinued for one hour during which time the solution became deepbrown-yellow, which indicates a colloid. The dark solution sometimes hada small amount of platinum particles which appeared. These can beremoved in subsequent steps. To be highly catalytic active, the colloidshould be activated.

Example 3

10 μl of a 8 wt. % platinum chloride solution in water (prepared from H₂PtCl₆, content˜4 wt. %) was added to 0.2M buffer. The solution washeated to approximately 90° C., and 100 μl of 25 wt. % glutaricdialdehyde was added: The heating was continued, and the solution wasmonitored for the appearance of the colloidal platinum as evidenced bythe formation of a brown color.

Example 4

10 μl of a 8 wt. % platinum chloride solution in water (prepared from H₂PtCl₆, Pt content˜4 wt. %) was added to 0.2M buffer. The solution washeated to approximately 90° C., and 100 μl of 37 wt. % formaldehyde wasadded. The heating was continued, and the solution was monitored for theappearance of colloidal platinum as evidenced by the formation of abrown color. Usually, precipitation of the colloid occurred.

Example 5--Mixed Metals.

10 μl of a 8 wt. % platinum chloride solution in water (prepared from H₂PtCl₆, Pt content˜4 wt. %) and 50 μl of 0.5 wt. % gold chloride wasadded to 0.2M buffer. The solution was heated to approximately 90° C.,and 100 μl of 10 mM ascorbic acid was added. The heating was continued,and the solution was monitored for the appearance of colloidal platinumas evidenced by the formation of a brown color. Catalytic activity wassimilar to that observed with pure platinum.

II. Activation of Particles

The colloids prepared in the above-described manner should be removedfrom any excess reducing agent to enhance their catalytic activity. Thismay be accomplished by a number of means which include: dialysis,oxidation by hydrogen peroxide, column chromatography, and precipitationwith washing. All these procedures produce colloidal particles withsimilar activities.

Activation By Hydrogen Peroxide

To the warm solution was added 1 mL of 30% hydrogen peroxide in 100 μlportions. Very rapid oxygen evolution was noted, as the platinum colloidis a very effective catalyst for the decomposition of hydrogen peroxide.The hydrogen peroxide removes some of the ascorbic acid and itsdecomposition products from the colloid surface and disperses thecolloid if partial collagation has occurred. After this treatment, thecolloid will readily coagulate unless a protective protein is added.

Activation by Dialysis

The protein to be coupled is added, and the solution is dialyzed againstdistilled water at 4° C. which removes more of the decompositionproducts of the ascorbic acid. Alternatively, the dialysis may bepreformed first and then the hydrogen peroxide added followed by theprotein. Either procedure gives comparable results, but the order mayaffect the activity of the protein in its intended usage, and the orderselected would depend upon the protein.

Activation by Precipitation

3-5 Volumes of isopropanol or ethanol are added to the colloid, and thecolloid is concentrated by centrifugation. The supernatant is discarded,and the colloid is resuspended in water. Without the isopropanol orethanol, no precipitation occurs, unless the colloidal particles arelarge and black in color, which usually indicates coagulation of thecolloids.

III. Attachment of Protein and Analysis

The protein is added to the solution of colloid and adsorbsnon-specifically. Due to the small particle size of most preparations ofthe colloids, it has not been possible to clearly separate the colloidalparticles from unadsorbed protein by either sizing columns, SDS gelelectrophoresis, agarose gel electrophoresis, or HPLC sizing columns.Some separation occurs, but the degree of separation is frequently notsufficient to clearly separate the bound from unbound protein.

On horizontal, 1.5% Agarose gels, the colloid typically produces twobands. One broad smear moves faster than Bovine Serum Albumin (BSA) andindicates either highly charged particles or very small particles. Theother band does not move at all and either indicates uncharged particlesor very large particles. Upon mixing with BSA, these bands combine intoone which moves slower than the original colloid and slightly fasterthan the unlabeled BSA. If insufficient protein is present to label allthe colloidal platinum particles, then three bands containing platinumappear, two of which are unlabeled platinum. Similar patterns areobserved on native, gradient polyacrylamide gels.

Some colloidal preparations appear to bind protein better than others asevidenced by the change in pattern on the Agarose gels. Both theascorbate-reduced platinum and the glutaric dialdehyde-reduced platinumbind more protein than the formaldehyde reduced-colloid. However, theformaldehyde-reduced materials also coagulate and may not migrate due tothe large size of the particles. The protein appears to be more stableon the glutaric dialdehyde-prepared colloids compared to the colloidalplatinum prepared with the other reducing agents. However, the stabilityappears to be related to the storage buffer. Phosphate buffered saline(PBS) pH 7.0 appears to be optimum, whereas in borate pH 8.0, mostimmunological recognition capability is lost after 3 days at 5° C. Thecatalytic activity of the colloid is stable which indicates that theadsorbed antibody is being displaced by the buffer. Also, analysis byelectrophoresis shows that the migration of particles with adsorbedproteins changes with time in some buffers probably due to aggregation.

Purification of Particles with Protein Prepared by the Ascorbate Method

After the adsorption of the protein, the excess protein is removed bychromatography on a Sephadex LH 150-120 prepared with phosphate bufferedsaline. The colloid elutes as a brown band and is well separated fromany yellow decomposition products of the ascorbic acid not completelyremoved in the dialysis step. Likewise, any coagulated platinum will beretained at the top of the column. However, the protein, BSA, alkalinephosphatase or IgG elute just slightly slower than the colloid, andthus, the separation is not complete. This colloid appears to beslightly larger than most proteins in size (see below).

Sizing of the Particles

The colloid prepared in the above manner with ascorbate is between 20 nmand 100 nm in size as determined by it completely passing through a 100nm Anotop membrane and being completely retained by a 20 nm Anotopmembrane. The retained material may be removed from the membrane andemployed in an immunoassay. This is an alternative method to using asizing column for the removal of unbound protein.

The colloid prepared in the above manner with glutaric dialdehyde isless than 20 nm in size as determined by it completely passing through a20 nm Anotop membrane with no loss in activity of UV-Visible absorption.

Various preparations were examined under an electron microscope. Theparticles were coagulated to varying degrees presumably due to thepreparation conditions necessary for electron microscopy. It is unclearif these particles were coagulated in solution, because in all casesthey readily passed through 100 nm filters but showed much greateraggregates under the electron microscope. In all cases, the particlescomposing the aggregates were less than 50 nm in diameter. Theresolution of the electron microscope was insufficient to determineclearly the topography of the individual particles.

IV. Use in a dot ELISA

Colloidal platinum was labeled with goat anti-biotin antibodies. EitherAntibodies or BSA, the carrier protein, labeled with biotin wereserially diluted in PBS and spotted onto nitrocellulose membranes. Theremaining active sites were blocked with 1% BSA in PBS. The membraneswere incubated with the colloidal platinum labelled with streptavidinfor varying periods of time and with varying amounts of colloid. In thecase of high amounts of carrier protein, a brown spot was barely visiblewhere the biotinylated protein was applied. With lesser amounts ofbiotinylated protein, no spot was visible due to the color of thecolloid. However, in all cases, when the nitrocellulose was placed in asolution of N,N-diethylphenylenediamine, 4-chloronapthol, and hydrogenperoxide, a strong blue precipitate appeared where the biotinylatedprotein had been applied. Also, the intensity decreased with decreasingamounts of biotinylated protein.

Colloidal platinum prepared with glutaric dialdehyde was labeled withstreptavidin. The amount of streptavidin was optimized by both observingthe pattern on Agarose gels and the sensitivity produced in a trialimmunoassay. Either antibodies or BSA, the carrier protein, labeled withbiotin were serially diluted in PBS and spotted onto nitrocellulosemembranes. The remaining active sites were blocked with 1 wt. % BSA inPBS. The membranes were incubated with the labeled colloidal platinumfor varying periods of time and with varying amounts of colloid. In thecase of high amounts of carrier protein, a brown spot was barely visiblewhere the biotinylated protein was applied. With lesser amounts ofcarrier protein, no spot was visible due to the color of the colloid.However, in all cases, when the nitrocellulose was placed in a solutionof N,N-diethylphenylenediamine, 4-chloronapthol, and hydrogen peroxide,a strong blue precipitate appeared where the biotinylated protein hadbeen applied. Also, the intensity decreased with decreasing amounts ofbiotinylated protein. Comparison with a commercial preparation ofstrepavidin-horseradish peroxidase conjugate incubated at the same timeshowed that the colloidal platinum was similar in sensitivity if notslightly better.

V. Optimization of catalytic activity

The pH, buffer, and hydrogen peroxide concentration optimum is differentfor the catalytic platinum particles as compared to horseradishperoxide. Generally, much higher concentrations of peroxide arenecessary to achieve rapid reaction, with the activity directlyproportional to the hydrogen peroxide concentration. Likewise, theactivity varies depending upon the buffer and substrate. A number ofbuffer systems, substrates and hydrogen peroxide concentrations weretested. The maximum hydrogen peroxide concentration used was 4 wt. %,although higher activity may be expected at higher concentrations. Twooptimal systems are MBTH-DMAB in 0.1M phosphate buffer, pH 4 andN,N-diethylphenylenediamine-4-chloronapthol in 0.1M borate pH 7. Thelatter system produces a water-insoluble dye that localizes at the siteof catalytic activity. The former system is for ELISA plate assays wherea water-soluble dye is desirable.

VI. Detection by Chemiluminescence

Lucigenin and luminol are dyes that may be used as the chemiluminescingmaterial in two different systems. Reactions catalyzed by platinum canbe easily detected by the luminol system (see Scheme 1). The reducingagent, hydrazine (H₄ N₂), reacts with platinum to produce nitrogen (N₂)and hydrogen peroxide (H₂ O₂). The H₂ O₂ then oxidizes the luminol,producing light. The lucigenin system has a slightly different chemicalpathway (see Scheme 2). The hydrazine reacts with the platinum andbreaks down into nitrogen (N₂) and hydrogen (H₂). The H₂ activates theplatinum, forming platinum hydride (Pt-H₂). The Pt-H₂ reduces thelucigenin to form a derivative. This derivative then reacts with oxygenand produces light. The formation of H₂ O₂ as an intermediary product inthe lucigenin system is not necessary. ##STR1## Materials and Procedures

The lucigenin reaction was tested on a spectrofluorometer with the lightsource off. The substrate contained a known amount of 0.2Mbuffer/ethanol solution, 4×10⁻³ M hydrazine, and 4×10⁻⁴ M lucigenin in a2000 μl cuvette. Then 5 μl of platinum colloid was added and theemission intensity was measured per unit of time.

Different buffers were tested including borate, carbonate, andphosphate, of pH 8, 9 10, and 11. Various reducing agents were usedother than hydrazine, for example sodium borohydride and formaldehyde.

The reactions were also assayed photographically. Trials were run inELISA plates that had eight rows of twelve 2000 μl wells. The outside ofthe wells were sprayed with silver or black paint so that the lightgiven off by one well would not stray to another well. 4×5 InstantPolaroid film was used for the photographs. The reagents in thereactions were varied as on the spectrofluorometer. The bottom two rowsof each plate were left blank, for the control.

These reactions were also assayed on nitrocellulose strips about threecentimeters in length and one centimeter in width. Several platinumcolloid dilutions were made in small test tubes, starting with 1:100,1:200, 1:400, 1:800, etc. 10 μl of BSA were added to each test tube tohelp the platinum stick to the nitrocellulose. 4 μl of each dilutionwere spotted onto the strips and were soaked in 0.4M borate buffer pH 8.A hole was drilled in the center of a 60 mm petri dish. The open end ofthe dish was covered with plastic wrap, which was held onto the platewith a rubber band. The strip and substrate were put into the hole andexposed on the film through the plastic wrap (this allowed closercontact with the film and sharper images than obtained through a thickplastic plate) for a specific length of time.

After the pictures were taken, the strips were developedcolorimetrically. The substrate for the color system contained ethanol,4-chloro-1-naphthol, N,N-diethylphenylenediamine dihydrochloride, 0.4Mborate buffer pH 8, and H₂ O₂. The intensity/density of the blue-coloredspots showed the amount of platinum present. Comparisons were madebetween the color and the light system to determine how well thechemiluminescence worked. Both methods gave sensitive readings of theamount of platinum colloid present.

The luminol system was also tested on a spectrofluorometer. Knownvolumes of 0.2M borate buffer, 4×10⁻³ M hydrazine, 5 mg/ml luminol, and10 mg/ml hematin were mixed together.

With the lucigenin system, improvements were made so that the emissionof light was more intense and longlasting. 5 μl of 2×10⁻³ M Pt added to950 μl of a 0.2M borate pH 9.5 buffer, 950 μl of ethanol, 100 μl ofhydrazine, and 10 μl of lucigenin in a 2000 μl cuvette yielded the bestresults on the spectrofluorometer. Generally, the phosphate buffer gavea light intensity that was much too low and the carbonate buffer gave ahigh background reading. There were problems with keeping the backgroundlow because when the pH was increased, the light emission went up, butunfortunately, so did the background. Similar problems were encounteredwhen adding a greater volume of lucigenin to the cuvette.

The lucigenin reaction gave off a large amount of light, but theintensity fell rapidly with time. It was difficult to determine why thelight intensity dropped so fast. One possibility was that some componentof the reaction was being consumed. The order in which the reagents wereadded was tested to see if that would affect the total chemiluminescenceintensity. When the hydrazine was added last, the luminescence was abouthalf the intensity of when the platinum or the lucigenin was added last.This implied that for some reason, the lucigenin was being consumed.Further investigation is being done to find a solution to this problem.

When the lucigenin system was tested on the nitrocellulose, a slightlydifferent combination of reagents gave the clearest light emission. Asolution containing 2 ml of 0.2M borate pH 9.5 buffer, 2 ml of ethanol,500 μl of hydrazine, 10 μl of lucigenin, and a small spatula scoop oftetrabutylammonium bromide worked the best. The tetrabutylammoniumbromide seemed to reduce the background light significantly because itreduced the non-specific adsoption of the lucigenin and the acridone tothe nitrocellose. The smallest amount of platinum that was detectablewas about 300 picograms.

The lucigenin system was also tested on the spectrofluorometer.Pilipenko et al disclose a system containing 1×10⁻⁴ M lucigenin, 0.02MH₄ N₂, sodium hydroxide (NaOH) at pH 13, and platinum concentrations of1×10⁻³ to 1×10⁻² (J. Analytical Chem. USSR, vol. 28, pp. 1004-8 (1971)).The concentration of platinum used was 2×10⁻³ M Pt. The light producedusing the reported system was useless, because the background noisealmost equaled the light intensity. With such a high background, it wasimpossible to test this reaction photographically because the lightemission could not be discerned from the background.

The lucigenin reaction gave a peak signal/background of 43,000/30 or12,000:1 and fell off over a period of thirty minutes. Although thebackground increased with time, it remained well below 30 units. Theseresults were much better, because with such a low background noiselevel, the light emission could be photographed successfully.

The luminol system was also tested with the spectrofluorometer. At a0.2M borate pH 11 buffer, the light intensity continued to rise, evenafter thirty minutes. However, results on the nitrocellulose strips werenot as good.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method for detecting an analyte in a sample,comprising:(i) contacting a sample which may contain an analyte with abiomolecule which is bonded to a catalytically active colloidal metalparticle, said biomolecule being a specific binding complement of saidanalyte, to obtain an analyte-biomolecule-colloidal metal particlecomplex; (ii) separating said analyte-biomolecule-colloidal metalparticle complex from said sample; (iii) contacting saidanalyte-biomolecule-colloidal metal particle complex with a substratewhich forms a product in a reaction catalyzed by the colloidal metalparticle of said complex; and (iv) detecting saidanalyte-biomolecule-colloidal metal particle complex by detecting saidproduct produced by a reaction of said substrate by the colloidal metalparticle of said complex to indicate the presence or absence of saidanalyte in the sample, wherein said colloidal metal particle comprises ametal selected from the group consisting of platinum, palladium, silverand mixture thereof; and said biomolecules is selected from the groupconsisting of antibodies, antigens, avidin, streptavidin, biotin,proteins bonded to a hapten, and nucleic acids.
 2. The method of claim1, wherein said colloid metal particle has a greatest dimension of lessthan 50 nm and a smallest dimension greater than 1 nm.
 3. The assay ofclaim 1, wherein said detecting is carried out by colorimetricallymeasuring the amount of said product catalytically produced by saidcolloidal metal particle.
 4. The method of claim 1, which is acompetitive assay.
 5. The method of claim 1, which is a sandwich assay.6. The method of claim 1, wherein said biomolecule is selected from thegroup consisting of antibodies, avidin, and streptavidin.
 7. The methodof claim 1, wherein said substrate comprises a mixture of H₂ O₂ and onemember selected from the group consisiting of hydrazine,1,2-dihydroxybenzene, dialkyl-N,N' phenylenediamine,1,3-dihydroxybenzene, 1,4-dihydroxybenzene, 1,2,3-trihydroxybenzene,ο-phenylenediamine, ο-methoxyphenol, coniferol, leucomalachite green,benzidine, ascorbic acid, guaiacol, diaminobenzidine, 5-aminosalicyclicacid, ο-dianisidine, the diammonium salt of2,2'-azino-di-(3-ethyl-benzthiazoline sulfonate-6),3-methyl-2-benzothiazolinone hydrazone, 3-(demthylamino)benzoic acid,and ο-tolidine.
 8. A method for detecting an analyte in a sample,comprising:(i) contacting a sample which may contain an analyte with abiomolecule which is bonded to a catalytically active colloidal metalparticle, said biomolecule being a specific binding complement of saidanalyte, to obtain an analyte-biomolecule-colloidal metal particlecomplex; (ii) separating said analyte-biomolecule-colloidal metalparticle complex from said sample; (iii) contacting saidanalyte-biomolecule-colloidal metal particle complex with a substratewhich forms light in a reaction catalyzed by the colloidal metalparticle of said complex; and (iv) detecting saidanalyte-biomolecule-colloidal metal particle complex by detecting saidlight produced by a reaction of said substrate catalyzed by thecolloidal metal particle of said complex to indicate the presence orabsence of said analyte in the sample, wherein said colloidal metalparticle comprises a metal selected from the group consisting ofplatinum, palladium, silver and mixtures thereof; and said biomoleculeis selected from the group consisting of antibodies, antigens, avidin,streptavidin, biotin, proteins bonded to a hapten, and nucleic acids. 9.The method of claim 8, wherein said biomolecule is selected from thegroup consisting of antibodies, avidin, and streptavidin.
 10. The methodof claim 8, wherein said substrate comprises a mixture of hydrazine anda member selected from the group consisiting of lucigenin and luminol.11. The method of claim 8, wherein said colloidal metal particle has agreatest dimension of less than 50 nm and a smallest dimension greaterthan 1 nm.
 12. The method of claim 8, which is a competitive assay. 13.The method of claim 8, which is a sandwich assay.
 14. A method fordetecting an analyte in a sample, comprising:(i) contacting a samplewhich may contain an analyte with a biomolecule which is bonded to acatalytically active colloidal metal particle, said biomolecule being aspecific binding complement of said analyte, to obtain ananalyte-biomolecule-colloidal metal particle complex; (ii) separatingsaid analyte-biomolecule-colloidal metal particle complex from saidsample; (iii) reacting said analyte-biomolecule-colloidal metal particlecomplex with hydrazine in the presence of lucigenin at a pH of 8 to 11;and (iv) detecting light generated by said reacting of saidanalyte-biomolecule-colloidal metal particle complex in the presence oflucigenin to indicate the presence or absence of said analyte in thesample; wherein said colloidal metal is selected from the groupconsisting of platinum, palladium, and silver; and said biomolecule isselected from the group consisting of antibodies, antigens, avidin,streptavidin, biotin, and proteins bonded to a hapten.
 15. The method ofclaim 14, wherein said biomolecule is selected from the group consistingof antibodies, avidin, and streptavidin.
 16. The method of claim 14,wherein said colloidal metal particle has a greatest dimension of lessthan 50 nm and a smallest dimension greater than 1 nm.
 17. The method ofclaim 14, which is a competitive assay.
 18. The method of claim 14,which is a sandwich assay.